bug#9903: 24.0.90; Empty input string signals error in batch mode.

[Chong Yidong]

1. emacs -Q -batch --eval '(message "%s" (read-string "ASDF: "))'
2. Type RET at the prompt, without entering any text.

Emacs signals an error:

Error reading from stdin

This problem was introduced by:

  revno: 105287
  committer: Paul Eggert <eggert@cs.ucla.edu>
  branch nick: trunk
  timestamp: Tue 2011-07-19 10:33:06 -0700
  message:
    Port to OpenBSD.

    See http://lists.gnu.org/archive/html/emacs-devel/2011-07/msg00688.html
    and the surrounding thread.
    * minibuf.c (read_minibuf_noninteractive): Rewrite to use getchar
    rather than fgets, and retry after EINTR.  Otherwise, 'emacs
    --batch -f byte-compile-file' fails on OpenBSD if an inactivity
    timer goes off.